A causal approach to disentangle fast and slow stratocumulus adjustments to aerosol perturbations

A large part of the uncertainty around future global warming is due to the cooling effect of aerosol-liquid cloud interactions, and in particular to the elusive sign of liquid water path (LWP) adjustments to aerosol perturbations. We quantify this adjustment with a novel causal approach that combines physical knowledge in the form of a causal graph with geostationary satellite observations of stratocumulus clouds. This allows us to remove confounding from large-scale meteorology and to disentangle counteracting physical processes such as cloud-top entrainment enhancement and precipitation suppression due to aerosol perturbations. The resulting LWP adjustment is time-dependent, with positive initial values due to fast precipitation suppression and negative values after entrainment enhancement has fully developed. We also use the causal framework as a diagnosis tool to detect potential issues with precipitation retrievals, which might cause precipitation-related influences on the LWP to be underestimated. These results suggest that time-aware causal analyses are key to reconcile conflicting studies concerning the sign of LWP adjustments across different data sources.